Coated steel sheet and method for manufacturing the same

Abstract

A coated steel sheet having a coated layer on surfaces of a steel sheet of a composition containing not less than 0.1 mass % and under 3 mass % of Al, wherein a following condition A or B is met: A: An AlN precipitate layer exists on a matrix side near an interface between said steel sheet and said coated layer B: Oxide of Al exists in said matrix right under said surfaces of said steel sheet.

1. A coated steel sheet having a coated layer on a surface of a steel sheet of a composition containing not less than 0.1 mass % and under 3 mass % of Al, wherein a following condition A or B is met:
 A: An AlN precipitate layer exists on a matrix side near an interface between said steel sheet and said coated layer 
 B: Oxide of Al exists in said matrix right under said surface of said steel sheet. 
 
     
     
       2. A coated steel sheet according to  claim 1 , wherein said coated layer is a hot-dip galvanized layer containing 0.1˜1.0 mass % of Al. 
     
     
       3. A coated steel sheet according to  claim 2 , wherein said coated layer is a Zn—Fe galvanneal coating that further contains 7˜15 mass % of Fe. 
     
     
       4. A coated steel sheet according to  claim 2 , wherein there is an AlN precipitate layer on the matrix side near an interface between said steel sheet and said coated layer and said AlN precipitate layer has a thickness of not less than 1 μm and not more than 100 μm. 
     
     
       5. A coated steel sheet according to  claim 2 , wherein said steel composition further contains one or two kinds selected from not less than 0.1 mass % of Si and not less than 0.5 mass % of Mn. 
     
     
       6. A coated steel sheet according to  claim 2 , wherein said steel composition further contains one or two kinds selected from not less than 0.01 mass % and not more than 1 mass % of Mo and not less than 0.005 mass % and not more than 0.2 mass % of Nb. 
     
     
       7. A coated steel sheet according to  claim 2 , wherein said steel composition further contains not less than 0.01 mass % and not more than 0.5 mass % of Cu, not less than 0.01 mass % and not more than 1 mass % of Ni, and not less than 0.01 mass % and not more than 1 mass % of Mo. 
     
     
       8. A coated steel sheet according to  claim 2 , wherein said steel composition further contains 0.03˜0.25 mass % of C, 0.001˜1.0 mass % of Si, 0.5˜3.0 mass % of Mn, and 0.001˜0.10 mass % of P. 
     
     
       9. A coated steel sheet according to  claim 8 , wherein in said steel matrix, there are one or more kinds selected from oxides of SiO 2 , MnO, FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Mn 2 SiO 4 , and P 2 O 5 . 
     
     
       10. A coated steel sheet according to  claim 8 , wherein the amount of oxides in total per one side surface is 0.01˜1.0 g/m 2 . 
     
     
       11. A coated steel sheet according  claim 8 , wherein the steel composition contains 0.01˜1.0 mass % of Mo and 0.005˜0.2 mass % of Nb. 
     
     
       12. A coated steel sheet according to  claim 8 , wherein the steel composition is 0.01˜0.5 mass % of Cu, 0.01˜1.0 mass % of Ni, and 0.01˜1.0 mass % of Mo. 
     
     
       13. A coated steel sheet according to  claim 8 , wherein the coated layer is a hot-dip galvanized layer and is galvannealed. 
     
     
       14. A coated steel sheet according to  claim 8 , wherein the Fe content in the galvannealed coated layer is 7˜15 mass %. 
     
     
       15. A method for manufacturing a coated steel sheet comprising the steps of heating and holding a steel slab and hot-rolling said slab, and hot-dip-galvanizing a steel sheet, wherein said slab contains not less than 0.1 mass % under 3 mass % of Al, and said holding is carried out in an atmosphere containing not less than 1 vol % and not more than 20 vol % of O 2  and not less than 70 vol % of N 2  under the conditions that meet an equation (1) shown below and said galvanizing is performed by using a galvanizing bath with an Al concentration in the bath is 0.14˜0.24 mass % at a bath temperature of 440˜500° C.
   {Heating and holding temp.(° C.)−(1050+25Al)}×heating and holding time (min)≧3000  (1) 
 
       wherein Al denotes an Al content (mass %) in the steel. 
     
     
       16. A method for manufacturing a coated steel sheet according to  claim 15 , wherein said steel sheet is galvanized by using a galvanizing bath of Al concentration of 0.10˜0.20 mass % in the bath at a bath temperature of 440˜500° C. and the hot-dip-galvanized layer is further subjected to a galvannealing process at 460˜550° C. 
     
     
       17. A method for manufacturing a coated steel sheet according to  claim 15 , wherein cold-rolling is carried out between the hot-rolling process and the hot-dip galvanizing process. 
     
     
       18. A method for manufacturing a coated steel sheet according to  claim 15 , wherein said steel slab further contains one or two kinds selected from not less than 0.1 mass % of Si and not less than 0.5 mass % of Mn. 
     
     
       19. A method for manufacturing a coated steel sheet according to  claim 15 , wherein said slab further contains one or two kinds selected from not less than 0.01 mass % and not more than 1 mass % of Mo and not less than 0.005 mass % and not more than 0.2 mass % of Nb. 
     
     
       20. A method for manufacturing a coated steel sheet according to  claim 15 , wherein said slab further contains not less than 0.01 mass % and not more than 0.5 mass % of Cu and not less than 0.01 mass % and not more than 1 mass % of Ni, and not less than 0.01 mass % and not more than 1 mass % of Mo.